Classical thermodynamics successfully describes the resulting macroscopic behavior without accounting for this underlying complexity, creating a disconnect between what we observe and what is actually happening. This chapter frames that gap between the microscopic world of atoms and the macroscopic world of thermodynamics—the gap this book aims to close.

Simplicity exists at the most fundamental level

It’s not complicated. It really isn’t.

Atoms move according to a small set of very basic laws. The difficulty is not the laws themselves—it’s the size of the system. Thermodynamic systems contain on the order of 10²³atoms. Because of this, because we can’t track this many atoms, we must rely on macroscopic properties such as temperature, pressure, and energy. There is no obvious, direct line connecting the two.

Motion begins with Newton

In his 1687 Principia, Sir Isaac Newton proposed three laws of motion, summarized here:

• 1st Law – an object maintains its motion unless acted upon by a force. If no forces act, motion continues unchanged—same speed, same direction.

• 2nd Law – forces change motion. A force acting on an object causes acceleration — speeding up, slowing down, or changing direction.

• 3rd Law – forces come in pairs. When one object exerts a force on another, the second object exerts an equal and opposite force on the first.

For a single atom, these rules are simple. For trillions of atoms, they are still simple—just repeated endlessly. Every thermodynamic phenomenon ultimately traces back to these laws applied at massive scale. Even modern molecular dynamics simulations are, at their core, Newton’s laws iterated billions of times.

What forces act on atoms?

As introduced in Chapter 1, two fundamental interactions are relevant here: gravitational and electromagnetic.

Gravitational

Every object attracts every other object through gravity. The force is always present—but at the atomic scale, it is negligible. Gravity matters when large masses are involved (like Earth), not when considering interactions between individual atoms.

Electromagnetic

This is where everything happens.

Atoms contain charged particles: positively charged protons and negatively charged electrons. Because of this separation of charge, atoms interact through electromagnetic forces—attraction and repulsion.

Unlike gravity, we don’t talk about electromagnetic forces in everyday life. But they are responsible for essentially all of chemistry, all material behavior, and most of what we experience as “physical reality.”

At a simplified, classical level, atoms can be thought of as tiny charge distributions interacting with one another. Modeling these interactions exactly is extremely difficult, so we often rely on empirical equations (such as the Lennard-Jones model). What matters for now is the qualitative behavior:

• Far apart → negligible interaction
• Moderate distance → net attraction
• Very close → strong repulsion

That’s it. Attraction and repulsion, varying with distance.

From this simple picture, everything else follows.

From motion to temperature

We now connect motion to thermodynamics.

Temperature reflects the average kinetic energy of molecular motion. Faster-moving atoms correspond to higher temperature; slower-moving atoms correspond to lower temperature.

We will formalize this later. For now, the physical understanding is sufficient: heating a system means increasing the energy of atomic motion.

From motion to pressure

Pressure is also a direct consequence of atomic motion.

Consider atoms moving inside a container at high speeds—on the order of 500 m/s (about 1,100 mph) for air molecules at room temperature. Each time an atom collides with the wall, it exerts a force. The accumulation of these countless impacts is what we measure as pressure.

• More atoms → more collisions → higher pressure
• Higher speeds → harder collisions → higher pressure

Pressure is not abstract. It is the summed effect of many, many atomic impacts.

As described in the Side Bar / Ideal Gas Law, this picture leads directly to the Ideal Gas Law: PV = nRT.

Why does the simplicity of attraction/repulsion lead to the complexity of thermodynamics?

It’s amazing how the seeming simplicity of the above theories and laws translates into the complexity of the behavior of moving and interacting atoms. Yes, these basic theories and laws lead to the Ideal Gas Law, and that’s all fine and good. One can understand it. The Law relies on directly measureable—and physically understandable—quanties of temperature, pressure, volume, and mass. But stray away from this ideal situation and you immediately run up against complexity. Here’s why.

The electromagnetic forces of attraction and repulsion never go away. They are always present.

In an ideal gas, intermolecular forces are negligible. Atoms move so quickly and remain so far apart that interactions don’t significantly affect their motion. Energy is essentially all kinetic. Potential energy plays no role.

But as temperature decreases, atoms move more slowly. They spend more time near one another. Now the attractive forces matter and atoms accelerate toward each other. Kinetic energy increases while potential energy, which had been near zero for the ideal state, decreases such that total energy remains constant. More on the details behind the energy transformations involved here will be addressed later.

In real systems, total energy is no longer purely kinetic. It is a combination of kinetic and potential contributions, and we cannot directly measure either at the atomic level. Instead, we rely on macroscopic properties like temperature and on conservation laws.

The bottom line: as soon as potential energy becomes significant, complexity increases rapidly.

Adding more layers

Now the situation evolves further:

• Condensation and phase change – As temperature drops and attractive forces increase, atoms begin to “capture” one another. Gases become liquids, and eventually solids.
• Distribution of speeds – At equilibrium, atoms do not all move at the same speed. There is a distribution: some fast, some slow, most near the average. Slower atoms are more susceptible to attraction, which can lead to phase coexistence (e.g., vapor–liquid systems).
• Mass matters – Temperature relates to kinetic energy: KE=1/2 mv². Thus, at the same temperature, lighter molecules move faster. For example, hydrogen (H₂) moves about 3.74 times faster than nitrogen (N₂).
• Mixtures and reactions – Introduce multiple species and chemical reactions, and the interaction network becomes even more complex.

At this point, tracking individual atoms becomes effectively impossible.

Why thermodynamics still works

And yet, it is rather remarkable that classical thermodynamics works perfectly well without tracking a single atom.

The founders built this science on macroscopic properties (temperature, pressure, energy, entropy) and exact relationships between them. They made no assumptions about atomic motion.

And that is both its strength and its weakness.

Students learn to use the equations, but often have no idea why they work. Thermodynamics becomes a black box.

• Why does a gas cool during Joule–Thomson expansion?
• Why does the Clausius–Clapeyron relation describe phase change so accurately?

These questions, among many others, expose the gap between the microscopic world and macroscopic theory.

This book is about closing that gap—showing how the simple rules governing atoms give rise to the full structure of thermodynamics.

Looking Ahead

This chapter has introduced the mechanical picture: atoms move according to Newton’s laws and interact through electromagnetic forces. From this motion arise temperature, pressure, and the other macroscopic properties we measure.

In later chapters we will build on this foundation by asking:

• How are the speeds of atoms distributed?
• Why do systems move toward equilibrium?
• How do energy exchanges at the atomic level lead to the laws of thermodynamics?

Understanding atomic motion is the first step toward answering those questions.

Summary

Atoms obey simple, deterministic laws of motion, interacting through electromagnetic forces of attraction and repulsion. Yet when these simple rules are applied across enormous numbers of atoms, they produce behavior of great complexity—far beyond what can be tracked or directly measured at the microscopic level.

Classical thermodynamics sidesteps this complexity entirely, describing systems through macroscopic properties and exact relationships that make no reference to atomic motion. This creates a fundamental disconnect: the equations work, but the underlying physical “why” is obscured. This chapter establishes that gap between the microscopic and macroscopic worlds, setting the stage for building the connection between them.

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